An on-grid solar hybrid system is expected to perform at considerably lower FCR than that of the fossil firing power plants, which feed the grid. Hence, renewable hybrid systems should not be recommended to run at a full annual load, unless generators with improved efficiencies are employed. For illustration, shown in the figure are three less steep, light dashed lines, which represent the use of CC of 60% for power generation only beyond the solar hours. Thus, the hybrid Rankine cycle will operate only at the solar regime (2000 hours), and the fossil fired Cc, only during the rest 6760 hrs of the year. In this illustration there is no integration between the CC and the Rankine cycle of the solar hybrid plant. They are separate. As expected, the FCR and GREF values with this arrangement come out better then before; and some environmental benefits (GREF) are kept for longer hours. The differences between the 3 modes are apparent. Other systems arrangements and operation modes and their related environmental benefits (as a function of the length of operation time) can be directly evaluated according to the system FCR and GREF by use of Figure 1.

The 60% level seems to offer a worthwhile standard for industrial and fast developing countries with advanced power systems and grids and availability of gas. On some cases relaxation of standards may be asked, such as for countries and regions without gas or particular constrains (distance from grid, low industrial readiness, or small system size). Thence the secondary standard of 40% may come to play. It represents many power plants which exist to day and which continue to spread worldwide. Whatever the standards taken for any GREF, it should always be transparent. It is obvious that a GREF related to a standard of 40% is numerically different from one related to a standard of 60%, which means also a difference in environmental contribution. The equations below quantify the GREF as a function of its specified standard and the conversion between different GREF outputs.

If a solar system is going to be added to an existing, off-grid diesel or Rankine-cycle generator at an isolated, remote site, it seems be reasonable to consider the new solar output as green energy (GREF=1), independently of the conversion efficiency of the existing generator. However, if we plan to install a new hybrid system, which includes a new fuel firing subsystem, it is mandatory to genuinely consider GREF and standards, in view of the global strategy for abating the global GHG (greenhouse gas). Implementation of the strategy requires frequent scrutiny of on-going decisions about variety of ways in using fuel. These
require the application of the yardsticks of FCR, GREF and CCA (cost of carbon avoidance, see below). Global strategy should encourage system refurbishment and installation of mini­grids at remote sites in order to enable improvement of generators efficiency. Fig. 1 and CCA (Equation 3) are useful for modelling off-grid solar hybrid systems as well.